1. Field of the Invention
The present invention relates to a radio communications system and a position detection method for achieving position detection of a mobile or fixed terminal.
2. Discussion of Background
A GPS (Global Positioning System) is well known as recent mobile communications systems represented by, for example, mobile phones. GPS is such that a GPS receiver receives radio waves from a plurality of GPS satellites to determine distances from the satellites and then to calculate a reception position of a terminal according to trilateration. A method using a DGPS (differential GPS) is also well known. This method is such that a fixed GPS receiver of a known position receives radio waves from GPS satellites to measure a measurement error by comparing detected positions and a known position and then to remove an error of a neighboring GPS recipient.
In the position detection method using GPS or DGPS, at least radio waves from GPS satellites need to be received, so that a position of a terminal cannot be detected where GPS signals cannot be received.
A position detection method not using GPS or DGPS is described in JP-A No. 247737/1997 (Patent Document 3). In this document, a trilateration method using estimated distances is disclosed. In this method, an electric field of signals received from a plurality of base stations of a mobile communications system is measured to estimate distances between the base stations and a terminal. Also in this document, another method for detecting a position of the terminal by determining positions of the base stations in communication with the terminal is disclosed. Unfortunately, In the position detection method using received electric field, large errors occur due to level fluctuation such as multipath fading and shadowing, thus degrading accuracy of the position detection.
JP-A No. 93650/1994 (Patent Document 1) is an example prior art reference for improving the measurement accuracy. In this document, electric field strength maps of received electric field levels are previously prepared to determine a position of the terminal from the maps of a plurality of base stations. Unfortunately, in this method, the maps are hard to generate, and need to be generated again when the neighboring layout changes.
JP-A No. 244967/2000 (Patent Document 2) is an exampled of another prior art reference for improving the measurement accuracy. In this document, since a distance estimated from the received electric field strength has a large error, the measurement accuracy is improved using the fact that a round-trip propagation time between the base station and terminal in communication with each other has a small error. Additionally, to improve an accuracy of the distance estimated from the received electric field strength, a parameter of the radio wave propagation is modified according to accurate position information determined using the GPS system.
Documents “Wireless LAN Integrated Access System (1)” B-5-203 by Ogino, et al. (Non-Patent Document 1) and “Wireless LAN Integrated Access System (2)” B-5-204, by Tsunehara, et al. (Non-Patent Document 2), from Proceedings of the General Conference of the Institute of Electronics, March 2003, Information and Communication Engineers, are exampled as further prior arts for improving the measurement accuracy. These documents describe that, by use of a wireless LAN (Local Area Network) system, a TDOA (time difference of arrival) from a terminal to each base station is measured to determine a position of the terminal according to trilateration. JP-A No. 244967/2000 describes that the error when a distance is estimated from the time-difference-of-arrival is smaller than the error when a distance is estimated from received power strength. The method of JP-A No. 244967/2000 uses the time difference of arrival only when the distance between the base station and the terminal in communication with each other is measured. In a position detection method according to TDOA using the wireless LAN system, the time difference of arrival between all the neighboring base stations and the terminal are measured to improve the accuracy of the position detection.
However, in the position detection method using the wireless LAN system, modulation of the time-differences-of-arrival between the base stations operated by asynchronous clocks and calculation of time-differences-of-arrival between all the base stations and the terminal are executed every measurement to determine the solution according to trilateration. As a result, many calculation processes are required to take a long time until the position detection.